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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/103008] poor inlined builtin_fmod on x86_64
Date: Thu, 10 Feb 2022 18:09:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103008

--- Comment #10 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
FYI, the following testcase:

--cut here--
#include <math.h>

float
__attribute__((noinline))
_fmodf (float x, float y)
{
  return x - truncf (x/y) * y;
}

int
main ()
{

  float a, b;
  volatile float z;

  for (a =3D -1000.0f; a < 1000.0f; a +=3D 0.01f)
    for (b =3D -1000.0f; b < 1000.0f; b +=3D 0.1f)
      z =3D fmodf (a, b);

  return 0;
}
--cut here--

$ gcc -Ofast -lm fmod-bench.c

      22,127092116 seconds time elapsed

      22,125111000 seconds user
       0,000999000 seconds sys


$ gcc -Ofast -fno-builtin-fmodf -lm fmod-bench.c

      32,751589079 seconds time elapsed

      32,746156000 seconds user
       0,000999000 seconds sys


Which points that the x87 code is considerably faster on my target
(Ivybridge-E) on Fedora-34 with glibc-2.33.

For reference, when the above _fmodf is called, I get:

$ gcc -Ofast -lm fmod-bench.c

      10,706189749 seconds time elapsed

      10,704859000 seconds user
       0,000999000 seconds sys

$ gcc -Ofast -lm -msse4 fmod-bench.c

      11,391062747 seconds time elapsed

      11,390771000 seconds user
       0,000000000 seconds sys

So, considerable faster!

It looks that with -ffast-math it is not inlined x87 code that is problemat=
ic,
but the missing fmod transformation. As shown above, the SSE2 code for trun=
cf
is on par with SSE4 roundss instruction, so if the target can provide optim=
ized
truncf code, the fmodf should definitely be converted to "a - trunc (a/p) *=
 p".=